selftests/bpf: Run SYN cookies with reuseport BPF test only for TCP

Currently we run SYN cookies test for all socket types and mark the test as
skipped if socket type is not compatible. This causes confusion because
skipped test might indicate a problem with the testing environment.

Instead, run the test only for the socket type which supports SYN cookies.

Also, switch to using designated initializers when setting up tests, so
that we can tweak only some test parameters, leaving the rest initialized
to default values.

Fixes: eecd618b ("selftests/bpf: Mark SYN cookie test skipped for UDP sockets")
Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200224135327.121542-2-jakub@cloudflare.com